Poor diagnostic for impl for T

[Ravenslofty]

I accidentally typed impl for T instead of impl T, and the diagnostic (playground) wasn't very helpful:

  |
3 | impl for T {}
  |          ^ expected `<` here

This appears to be because of this syntax:

impl for<'a> T

However it makes no sense in this case. Perhaps better diagnostics would look something like this:

  |
3 | impl for T {}
  |     ^ expected type

[troiganto]

Maybe relatedly, this code compiles on stable 1.30.0:

trait Trait {}

impl for<'a> Trait {}

fn main() {}

(Playground link)